Conventionally, there is known a multilayer circuit board in which two wirings formed on an insulating resin layer are connected to each other. As an interlayer connecting method, it is known to provide a via hole conductor formed by filling a hole formed on an insulating resin layer with a conductive paste.
In the case in which the conductive paste is used as the via hole conductor, the conductive paste in an uncuring state in the via hole conductor is cured together with a prepreg in semi-curing state. For this reason, the curing of the conductive paste is influenced by a resin contained in the prepreg (softening, flow or the like) in some cases.
FIGS. 22A to 24C are typical sectional views showing a conventional method of manufacturing a circuit board. With reference to FIGS. 22A to 24C, description will be given to the conventional method of manufacturing a circuit board.
As shown in FIG. 22A, first of all, protective film 1 is formed on both surfaces of prepreg 2.
As shown in FIG. 22B, next, hole 3 is formed on prepreg 2 and protective film 1.
As shown in FIG. 22C, subsequently, squeegee 5 is moved in a direction of arrow 6 over protective film 1, and hole 3 formed on prepreg 2 is filled with via paste 4.
As shown in FIG. 23A, then, protective film 1 is peeled so that copper foil 8 is provided on the both surfaces of prepreg 2 having a part of via paste 4 set to be protruded portion 7 and they are pressurized, heated, and integrated as shown in arrow 6.
As shown in FIG. 23B, thereafter, via 11 and insulating layer 10 are formed by curing via paste 4 and prepreg 2, respectively.
As shown in FIG. 23C, next, copper foil 8 fixed to a surface of insulating layer 10 is subjected to etching to form wiring 12. Clearance 14 is present between a plurality of wirings 12 provided on a surface of core substrate portion 13.
As shown in FIG. 24A, subsequently, prepreg 2 provided with protruded portion 7 constituted by via paste 4 and copper foil 8 are laminated on wiring 12 provided on the surface of core substrate portion 13.
As shown in FIG. 24B, then, a part of prepreg 2 flows as shown in arrow 6b to fill in clearance 14 between wirings 12. With reference to FIG. 24C, further detailed description will be given to a portion shown in additional line 15 of FIG. 24B.
FIG. 24C shows a state in which a part of via paste 4 flows so that deformed via 16 is formed when a part of prepreg 2 heated and softened flows as shown in arrow 6b to fill in clearance 14 between wirings 12.
In order for prepreg 2 softened by the heating to cause via paste 4 to flow, clearance 14 between wirings 12 is narrowed by a via (deformed via 16) flowing in from both sides as shown in arrow 6b. For example, an interval shown in arrow 6d is narrowed as shown in arrow 6c. 
In order to meet a demand for thinning a circuit board, it is desirable that prepreg 2 should be thinned. When prepreg 2 is thinned more greatly, however, it is harder to cause a portion between wirings 12 to be flat. For this reason, a further fluidization of prepreg 2 is required. When the circuit board is thinned more greatly, and furthermore, a pattern is made finer or an adjacency is enhanced more greatly, via paste 4 is more deformed. As a result, an occurrence probability of deformed via 16 is increased.
When the circuit board thus has a finer pith (including a decrease in a via diameter, a reduction in an adjacency between the vias and the like), there is increased a possibility that the deformation of the via caused by the via paste might influence an insulating property between the wirings, a migration characteristic, a via resistance and a variation in the via resistance.
Against the problems, it is proposed to drop a curing start temperature of a conductive paste to be lower than a melting start temperature of an insulating sheet. As the citation list, for example, PTL 1 is known.